Corolla of the Large-flowered Petunia (Petunia hybrida Vilm.) Cultivars Exhibit Low Endogenous Cytokinin Concentration through Enhanced Expression of the Genes Encoding Cytokinin Oxidases

Hormonal Signaling during dPCD: Cytokinin as the Determinant of RNase-Based Self-Incompatibility in Solanaceae

Research into molecular mechanisms of self-incompatibility (SI) in plants can be observed in representatives of various families, including Solanaceae. Earlier studies of the mechanisms of S-RNase-based SI in petunia (Petunia hybrida E. Vilm.) demonstrate that programmed cell death (PCD) is an SI factor. These studies suggest that the phytohormon cytokinin (CK) is putative activator of caspase-like proteases (CLPs). In this work, data confirming this hypothesis were obtained in two model objects-petunia and tomato (six Solanaceae representatives). The exogenous zeatin treatment of tomato and petunia stigmas before a compatible pollination activates CLPs in the pollen tubes in vivo, as shown via the intravital imaging of CLP activities. CK at any concentration slows down the germination and growth of petunia and tomato male gametophytes both in vitro and in vivo; shifts the pH of the cytoplasm (PHc) to the acid region, thereby creating the optimal conditions for CLP to function and inhibiting the F-actin formation and/or destructing the cytoskeleton in pollen tubes to point foci during SI-induced PCD; and accumulates in style tissues during SI response. The activity of the ISOPENTENYLTRANSFERASE 5 (IPT5) gene at this moment exceeds its activity in a cross-compatible pollination, and the levels of expression of the CKX1 and CKX2 genes (CK OXIDASE/DEHYDROGENASE) are significantly lower in self-incompatible pollination. All this suggests that CK plays a decisive role in the mechanism underlying SI-induced PCD.

miR156/157 Targets SPLs to Regulate Flowering Transition, Plant Architecture and Flower Organ Size in Petunia

miR156/157 plays multiple pivotal roles during plant growth and development. In this study, we identified 11 miR156- and 5 miR157-encoding loci from the genome of Petunia axillaris and Petunia inflata, designated as PaMIR0156/157s and PiMIR0156/157s, respectively. Real-time quantitative reverse transcription PCR (qRT-PCR) analysis indicated that PhmiR156/157 was expressed predominantly in cotyledons, germinating seeds, flower buds, young fruits and seedlings. PhmiR156/157 levels declined in shoot apical buds and leaves of petunia before flowering as the plant ages; moreover, the temporal expression patterns of most miR156/157-targeted PhSPLs were complementary to that of PhmiR156/157. Ectopic expression of PhMIR0157a in Arabidopsis and petunia resulted in delayed flowering, dwarf plant stature, increased branches and reduced organ size. However, PhMIR0156f-overexpressing Arabidopsis and petunia plants showed only delayed flowering. In addition, downregulation of PhmiR156/157 level by overexpressing STTM156/157 led to taller plants with less branches, longer internodes and precocious flowering. qRT-PCR analysis indicated that PhmiR156/157 modulates these traits mainly by downregulating their PhSPL targets and subsequently decreasing the expression of flowering regulatory genes. Our results demonstrate that the PhmiR156/157-PhSPL module has conserved but also divergent functions in growth and development, which will help us decipher the genetic basis for the improvement of flower transition, plant architecture and organ development in petunia.

Genome-Wide Identification, Characterization and Expression Analysis of TCP Transcription Factors in Petunia

The plant-specific TCP transcription factors are well-characterized in both monocots and dicots, which have been implicated in multiple aspects of plant biological processes such as leaf morphogenesis and senescence, lateral branching, flower development and hormone crosstalk. However, no systematic analysis of the petunia TCP gene family has been described. In this work, a total of 66 petunia TCP genes (32 PaTCP genes in P. axillaris and 34 PiTCP genes in P. inflata) were identified. Subsequently, a systematic analysis of 32 PaTCP genes was performed. The phylogenetic analysis combined with structural analysis clearly distinguished the 32 PaTCP proteins into two classes—class Ι and class Ⅱ. Class Ⅱ was further divided into two subclades, namely, the CIN-TCP subclade and the CYC/TB1 subclade. Plenty of cis-acting elements responsible for plant growth and development, phytohormone and/or stress responses were identified in the promoter of PaTCPs. Distinct spatial expression patterns were determined among PaTCP genes, suggesting that these genes may have diverse regulatory roles in plant growth development. Furthermore, differential temporal expression patterns were observed between the large- and small-flowered petunia lines for most PaTCP genes, suggesting that these genes are likely to be related to petal development and/or petal size in petunia. The spatiotemporal expression profiles and promoter analysis of PaTCPs indicated that these genes play important roles in petunia diverse developmental processes that may work via multiple hormone pathways. Moreover, three PaTCP-YFP fusion proteins were detected in nuclei through subcellular localization analysis. This is the first comprehensive analysis of the petunia TCP gene family on a genome-wide scale, which provides the basis for further functional characterization of this gene family in petunia.

Effect of extending the photoperiod with low-intensity red or far-red light on the timing of shoot elongation and flower-bud formation of 1-year-old Japanese pear (Pyrus pyrifolia)

To investigate the effects of light quality (wavelength) on shoot elongation and flower-bud formation in Japanese pear (Pyrus pyrifolia (Burm. f.) Nakai), we treated 1-year-old trees with the following: (i) 8 h sunlight + 16 h dark (SD); (ii) 8 h sunlight + 16 h red light (LD(SD + R)); or (iii) 8 h sunlight + 16 h far-red (FR) light (LD(SD + FR)) daily for 4 months from early April (before the spring flush) until early August in 2009 and 2010. In both years, shoot elongation stopped earlier in the LD(SD + FR) treatment than in the SD and LD(SD + R) treatments. After 4 months of treatments, 21% (2009) or 40% (2010) of LD(SD + FR)-treated trees formed flower buds in the shoot apices, whereas all the shoot apices from SD or LD(SD + R)-treated plants remained vegetative. With an additional experiment conducted in 2012, we confirmed that FR light at 730 nm was the most efficacious wavelength to induce flower-bud formation. Reverse transcription-quantitative polymerase chain reaction revealed that the expression of two floral meristem identity gene orthologues, LEAFY (PpLFY2a) and APETALA1 (PpMADS2-1a), were up-regulated in the shoot apex of LD(SD + FR). In contrast, the expression of a flowering repressor gene, TERMINAL FLOWER 1 (PpTFL1-1a, PpTFL1-2a), was down-regulated. In addition, expression of an orthologue of the flower-promoting gene FLOWERING LOCUS T (PpFT1a) was positively correlated with flower-bud formation, although the expression of another orthologue, PpFT2a, was negatively correlated with shoot growth. Biologically active cytokinin and gibberellic acid concentrations in shoot apices were reduced with LD(SD + FR) treatment. Taken together, our results indicate that pear plants are able to regulate flowering in response to the R : FR ratio. Furthermore, LD(SD + FR) treatment terminated shoot elongation and subsequent flower-bud formation in the shoot apex at an earlier time, possibly by influencing the expression of flowering-related genes and modifying plant hormone concentrations.